Effects of SiNx refractive index and SiO2 thickness on polarization-type potential-induced degradation in front-emitter n-type crystalline-silicon photovoltaic cell modules

Abstract

This study investigated how the SiNx refractive index (RI) and SiO2 thickness, dox, of stacked SiNx/SiO2 passivation layers of the front p+emitters of n-type crystalline-silicon (c-Si) photovoltaic (PV) cells affect their polarization-type potential-induced degradation (PID) behaviors. We prepared six n-type c-Si PV cells with an RI of 2.0 or 2.2 and with dox of 9, 2, or 1 nm. Then PV modules fabricated from the cells were subjected to PID tests during which a bias of −1000 V was applied to cells with respect to the front cover glass surface. For dox of 9 or 2 nm, rapid polarization-type PID was observed, irrespective of the RI. However, for dox of 1 nm, the RI markedly affected the degradation behavior, and cells with an RI of 2.2 showed no degradation. These findings are attributable to carrier transport between the high RI (Si-rich) SiNx and the c-Si substrates, which can readily occur only when the SiO2 layer is sufficiently thin for electrons to tunnel through the SiO2 layer. These results are important for elucidating polarization-type PID mechanisms and for developing preventive measures against polarization-type PID.